Factors and pathways in the pathogenesis of Graves' eye disease were investigated using cultures of fibroblasts derived from human retrobulbar tissue. Stimulation of hyaluronic acid (HA) production in the cultures may be analogous to the in vivo disease. Lymphocytes, from patients with Graves' eye disease and from normal subjects, stimulated HA synthesis; the process appeared to involve the adenyl cyclase system since 1) theophylline potentiated the stimulation, 2) dibutyryl cAMP was a potent stimulator, and 3) lymphocytes increased intracellular concentrations of cAMP in the fibroblast cultures. Prostaglandins, especially PGE1 and PGE2, also stimulated HA production; these factors may play a modulating role in the increased fibroblast activity. Stimulation was inhibited by cycloheximide and actinomycin D, but not cytosine arabinoside. Certain preparations of thyrotropin and luteinizing hormone stimulate HA synthesis in the cultures in patterns similar to those of lymphocytes. However, the active factor in the preparations may not be any known hormone.